The Blues have been linked with the Argentine in recent months, with Jose Mourinho believed to be a big fan of the rugged centre-back.
A move for his signature at the end of the season has been mooted, with the Portuguese champions resigned to losing him.
But, now Zenit have emerged as candidates in the race for the 25-year-old, and are keen to tie up a deal.
The Russian giants are ready to make a January move for Otamendi, which may appeal to Porto who are well aware that as the South American’s contract ticks towards its end in 2015, his value will decrease.
Zenit boss Luciano Spalletti has made strengthening his rearguard a priority heading into the next transfer window, and is willing to dedicate a large chunk of his budget to doing so.
The Russians have the financial clout to compete with Chelsea due to their significant backing from wealthy gas company, Gazprom.
AC Milan are also thought to be in the running.
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